1. Field
Embodiments described herein relate generally to a power saving control technique in a personal computer including a function of always supplying power to an external device connected by means of a Universal Serial Bus (USB) connector.
2. Related Art
In recent years, notebook and desktop personal computers have come into widespread use as tools providing various functions such as sending and receiving electronic mail (e-mail) over the Internet, browsing publically available information on the World Wide Web, and creating documents. These kinds of personal computer include various connectors for connecting external devices as the need arises.
For example, a USB connector is provided with a contact for a data line and a contact for a power supply line. Therefore, power is supplied from the personal computer, and thereby, an external device is operable.
Recently, mobile devices individually operable by a battery, such as portable music players and cellular phones, have come into widespread use. Many of these mobile devices are configured so that they are connectable to a USB connector for data exchange with a personal computer. These mobile devices configured to be connectable to a USB connector are mostly operable by power from the personal computer (without consuming their own battery). Further, the batteries of the foregoing mobile devices are mostly chargeable by power from the personal computer.
Moreover, a personal computer is widely used to connect a mobile device to a USB connector for the purpose of charging the battery of a mobile device. The foregoing use has attracted interests, and thus, the following personal computer has been proposed (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2006-53748). The personal computer is capable of always supplying power to a mobile device connected to a USB connector without turning on power, that is, in the power-off state.
A personal computer is capable of taking the following three states as a power-off state. One is a standby state of continuing the supply to a main memory and continuing to hold a work interruption state in a power-off state. Another is a hibernation state of migrating the content of a main memory to a hard disk drive (HDD) and stopping the supply to the main memory. Another is a shutdown state of abandoning the content of a main memory and stopping all supply. The foregoing three mode power-off states are expressed as “S3”, “S4” and “S5” to make a distinction between these three states according to Advanced Configuration and Power Interface (ACPI). In this case, a power-on state is expressed as “S0”.
A recent personal computer is required to support a wakeup function when being in a standby state. According to the wakeup function, when receiving a wakeup signal from an external device connected to a USB connector, the personal computer automatically returns to a power-on state. Therefore, the supply to a controller (i.e., USB host controller) for controlling an exchange with an external device connected to a USB connector must be continued when a personal computer is in a standby state even if it is in a power-off state. In other words, if the personal computer is in a hibernation state or shutdown state, it is possible to stop the supply to a USB host controller.
On the other hand, users connect an external device to a USB connector of a personal computer being in a power-off state to charge a battery. In this case, they do not recognize the difference between a standby state, a hibernation state and a shutdown state. Thus, in a power-off state, the personal computer supports a function of always supplying power to an external device connected to a USB connector (hereinafter, referred to as a sleep-and-charge function). In this case, the computer must continue the supply to a USB host controller in a hibernation state or a shutdown state.
For example, a notebook personal computer includes various designs for power saving in order to make a drivable time using power from (its own) battery as long as possible. For this reason, a mechanism for reducing power consumption as possible as can is required to support the foregoing sleep-and-charge function.